This invention relates to optical instruments and in particular to optical instruments having an exit pupil at which an image of an object may be viewed by an observer.
In conventional optical instruments the exit pupil is of relatively small dimension. Consequently it is necessary for an observer to accurately align the entrance pupil of his eye with the exit pupil of the optical instrument in order properly to view an image or otherwise receive light from the instrument.
It would be an advantage if the necessity for an observer to accurately align the entrance pupil of his eye with a relatively small exit pupil of the instrument were removed by providing the optical instrument with an enlarged exit pupil. The observer would then be able to enjoy the freedom to receive such images or light by placing his eye pupil anywhere within an enlarged exit pupil. However the size of the exit pupil of conventional optical instruments is determined by a function of the numerical aperture of the instrument and the overall magnification of the instrument and hence the size of the exit pupil is of fixed and relatively small dimension.